if i'm doing thrusters w/ kb's, i'll vary it up. sometimes i'll cup both hands around the bell, sometimes by the horns, sometimes i'll do one arm thrusters and alternate. the one arm thrusters is pretty interesting because it teaches the corkscrew motion that goes on during presses between your lower and upper body.
i have a 1, 1.5, and 2 pood. i find that if i really need make the motions heavier, i just do OL instead. would i like to have a pair of each? definately, but due to conveniance, money, and space (i keep my kb in a small locker at my school gym, they look pretty ridiculous in there) the single set is quite adequate. the reason that i like having the 2 pood as opposed to a pair of 1 pood and 1.5 pood is for heavy TGUs. i haven't really tried heavy TGUs with a barbell, but the 2 pood is normally ample enough to stimulate the motion well. plus i've had to drop the bell on these before.
honestly though, if you get 2 pairs or a set of 3 diff ones, you're gonna get nearly the same benefits. thus i wouldn't really worry which one you pick. you might have a stronger TGU with a 2 pood while you might have a stronger clean with a pair of 1.5, but are these differences that big a deal? it all really depends on what your goal is.